Usually, booms of lattice-boom cranes comprising a luffing tip or a fly jib are erected with the aid of a so-called superlift device with a counter boom and an additional counter weight or with the aid of an auxiliary crane, as it is e.g. already explained in the DE 30 42 287 A1 as being disadvantageous. To overcome this disadvantage, a boom for a telescopic crane is proposed there, the luffing tip jib of which has two tension supports being pivoted thereto, whereby the telescopic boom including the luffing tip can be mounted to a large extent lying on the ground and can be brought to the operating position subsequently without further auxiliary means. This is carried out in that during erecting the telescopic boom, the entire restraint is tightened, whereby the tension supports are erected. As soon as the restraint is tightened, the luffing tip jib is positioned in its final inclination with respect to the telescopic boom which is determined by the respective length of the guy wires. By further lifting the telescopic boom, the entire boom eventually is brought into its operating position.
Such a procedure, according to which the entire boom together with a luffing tip being arranged at the jib is erected in its operating position already, can usually only be realized, however, for very light and short luffing tip jibs. Usually, these luffing tips are extensions of the main boom of a crane, wherein the inclination of these luffing tips is adjustable during operation of the crane. In case one would try to erect the mostly very long booms of lattice-boom cranes, which if necessary can also be built in two pieces, i.e. comprising a main boom and a fly jib or luffing tip being jointed to the main boom, already in their operating position without any auxiliary means, normally this would not function due to the tilting moment producible by the weight of the boom in connection with the very long lever arm of the entire boom. This would normally fail because the tilting moment producible by the weight of the boom in connection with the very long lever arm of the boom usually is larger than the retaining standing moment of the mobile crane, which is determined by the geometric properties of the under carriage as well as the upper carriage and the weight distribution including additional counter weights.
To overcome said deficiency, the Japanese patent applications JP 11-322278 and JP 2002-46981 propose to provide the jibs of very weak lattice-boom cranes with an additional luffing fly jib or with a luffing tip, which during erection of the lattice-boom or telescopic jib bends relative to the boom. A similar lattice-boom crane is disclosed in U.S. Pat. No. 3,794,184. As it will become apparent from said document, the cranes disclosed therein exclusively relate to cranes comprising a main boom to which a separate fly jib is jointed, wherein the inclination of said fly jib is adjustable during operation as well. A similar crane is for example disclosed in the book “Das groβe Buch der Fahrzeugkrane” by Rudolf Becker on pages 88cf. as well as on pages 178cf. As will become apparent therefrom, the jib of said crane consists of two individual jibs, namely of a separate main boom and a separate fly jib which is luffably (arranged in a luffable manner) arranged to the main boom, both of which are adapted to luff with respect to each other during operation. Moreover, EP 0 733 584 discloses a tower crane comprising a needle jib, which is adapted to be folded up.
Indeed, by means of the two-piece construction of the jib known from JP 11-322278 and JP 2002-46981, the length and the height respectively as well as the reach which may be achieved by said jib may be increased. However, said supposed positive effect of such a jib consisting of a main boom and a separate fly jib, which is, even during operation, luffably arranged at the main boom, has an adverse effect on the working load of the jib, which rapidly decreases as a result of the articulated joint of the fly jib. Hence, these cranes are not suitable to achieve high working loads, wherefore there is usually no other choice but to use a lattice-boom crane having a rigid and unbending main boom, which normally only can be erected with the aid of an additional auxiliary crane, as will be illustrated below.
Hence, the skilled person is still often confronted with the problem, either to use relatively weak cranes having for example a two-piece jib comprising for example a separate telescopic jib and a separate fly jib, which is even during operation luffably arranged at the main boom, to serve major height by the crane. Because of the decreased working load caused by the two-piece construction of the jib, however, by means of these cranes it is not possible to carry major loads. Alternatively it was only possible to use heavier lattice-boom cranes, wherein the main boom of said cranes had to be erected with the aid of an auxiliary crane.
At this point the present invention intervenes, according to which the main boom of such a lattice-boom crane is no longer built up as a one-piece rigid jib, but according to which the main boom during assembly comprises two separate and individual lattice-boom portions being hinged to each other, which are adapted to become bended to each other during assembly and which are further adapted to become rigidly attached to each other during the operation of the crane or in the final state. Hence, by means of the present invention, the respective advantages of the aforementioned cranes can by combined with those advantages of a lattice-boom crane.
As can be deducted from the explanations above, within the scope of the present invention, by tilting moment, the moment is meant, which is generated by the boom as soon as the latter is supported only on one point, namely the point of the articulated joint at the upper carriage of a crane. As long as the boom has a second point of support besides this first point of support, no tilting moment is produced by this boom. On the other hand, however, by standing moment, the moment counter acting the tilting moment is meant. The standing moment is determined by the geometric properties as well as by the weight distribution of the under carriage, the upper carriage as well as possibly by the auxiliary and counter weights attached thereto.
The longer a boom is the larger is the tilting moment which can be generated thereby. To erect a very long boom at one time, it would be necessary to provide a large standing moment accordingly such that the tilting moment does not exceed the standing moment during the erecting process. In case the tilting moment should still be larger than the standing moment, the crane would at least tend to tilt. In the most disadvantageous case the crane would in fact tip over. To antagonize this risc, the under carriage and the upper carriage including all possible additional weights would have to be constructed such that the standing moment always is at least equal to the tilting moment producible. For very long booms, this however would lead to extraordinary heavy and large dimensions of the parts defining the standing moment, as for example the under carriage, the upper carriage as well as the weights attached thereto.
To take account of this, usually auxiliary cranes are used as illustrated above during the erection of long main booms or lattice-boom cranes, by means of which the main boom is brought into position. This, e.g., results in that the hook of the auxiliary crane is attached approximately in the center of the main boom. Subsequently, the main boom jointed to the upper carriage is erected with the auxiliary crane up to such an angle position, in which the tilting moment is smaller than the standing moment of the crane. As soon as this position is reached, a retaining cable of the crane is activated, keeping the main boom in this inclined position, such that the hook of the auxiliary crane can be disengaged.
In this position, the main boom is already in a crane operating position, in which the main boom can be swung only slightly around the hinge point on the upper carriage. In this crane operating position, in which the main boom mostly has a very steep inclination with respect to the horizontal plane, the tilting moment is essentially smaller than in the starting position, in which the entire main boom lies on the ground. This results from the fact that the respective tilting moment results from the boom weight and the projection of the boom length into the horizontal plane, which usually is also termed as effective lever arm. Thus, in the crane operating position, the main boom is inclineable to such a degree, according to which the effective lever arm is attached so far that the tilting moment does not exceed the standing moment.
Another known possibility to erect a long main boom is that special auxiliary means for erecting the main boom are used which mostly are attached to the crane as external means. Amongst these, e.g., is the so called super lift device, consisting of a restraint with a counter boom and an auxiliary counter weight which is arranged on the super lift device. The use of such a super lift device, however, requires an increased transport and time expenditure for the assembling and disassembling of the counter boom as well as the additional counter weights.
Starting from these known methods for erecting or disassembling a main boom, respectively, therefore, the present invention has the object to provide a method for erecting and disassembling a main boom for a lattice-boom crane, enabling to erect the main boom without an auxiliary crane or other auxiliary means, such that the time expenditure for the assembling and disassembling of the main boom is reduced and additional costs arising from the use of an auxiliary crane can be avoided. Further, according to the present invention, a main boom for a lattice-boom crane should be provided, which is erectable more easily and faster than known main booms and which combines the advantages of a common lattice-boom with the advantages of a telescopic crane assembled with an articulated and luffing fly jib.